Topic: Fluids
Concepts: General Newton’s 2nd law, Statics

Solution:

First, we study the force on one U-tube:

To find the force the stream of water exerts on the U-tube, we consider a small time interval $\Delta t$ where a small mass $\Delta m$ of water enters from the left at the top of the U-tube. Since the water is flowing at velocity $v$, the length of this segment is $\Delta L=v\Delta t$ so

At the same time, we have $\Delta m$ of water leaving to the left with velocity $-v$ at the bottom of the U-tube. Thus, in time $\Delta t$, water of mass $\Delta m$ effectively has its velocity changed from $v$ to $-v$. This change in momentum can be attributed to the force from the U-tube on the water,

By Newton’s 3rd law, this is also the force the water exerts on the U-tube. Going to the tube assembly of two U-tubes, since the net force is zero, we balance forces from each side:

so the answer is C.